Electronic displays using optically pumped luminescent semiconductor nanocrystals

ABSTRACT

A multicolor electronic display is based on an array of luminescent semiconductor nanocrystals. Nanocrystals which emit light of different colors are grouped into pixels. The nanocrystals are optically pumped to produce a multicolor display. Different sized nanocrystals are used to produce the different colors. A variety of pixel addressing systems can be used.

This application claims the benefit of U.S. Provisional No. 60/087,883filed Jun. 6, 1998.

This invention was made with U.S. Government support under Contract No.DE-AC03-76SF0098 between the U.S. Department of Energy and theUniversity of California for the operation of LAWRENCE BERKELEY NATIONALLABORATORY (LBNL). The U.S. Government may have certain rights to thisinvention.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates generally to electronic displays and moreparticularly to multi-color electronic displays based on luminescentsemiconductor nanocrystals.

2. Description of the Related Art

Flat panel display technologies are currently displacing cathode raytube (CRT) displays. CRT's main disadvantages are volume, weight andpower consumption. On the other hand, CRT's image quality, resolution,and color are still unsurpassed. An alternative technology, which hasalready made it to the market, is that of liquid crystal displays(LCDs).

In liquid crystal displays, an image is produced by turning “on” or“off” pixels by selectively passing or blocking light from a backlight.This is done with the help of sheet polarizers and by controlling thepolarization state of the light as it traverses the liquid crystal. In acolor display, each screen pixel is actually made of three separatedots, each with a respective red, green, or blue filter printed on theglass in front of it (a fourth white dot may also be included to adjustcontrast). These three primary colors are mixed in various amounts toform the variety of colors the user sees. Since the backlight isunpolarized and its spectrum is broad, a considerable amount of light isdissipated on the sheet polarizers and color filters, making thesedisplays energy inefficient.

Other important flat panel display technologies which are currentlybeing developed are field emitters, plasma displays and multi-colorsemiconductors and polymeric light emitting diodes based onelectroluminescence and/or optical pumping.

A nanocrystal (or nanometer crystal) is an organic or inorganic singlecrystal particle having an average cross-section no larger than about 20nm (200 Angstroms), and preferably no larger than about 10 nm (100 A)and a minimum average cross-section of about 1 nm or in some instanceseven a smaller average cross-section, i.e. down to about 0.5 nm (5 A).Typically the nanocrystals will have an average cross-section ranging insize from about 1 nm (10 A) to about 10 nm (100 A). A semiconductornanocrystal is a nanocrystal of group II-VI (e.g. CdS, CdSe, CdTe, ZnS,ZnSe, ZnTe), or group III-V (e.g. GaAs, InAs, InGaAs, InP) semiconductorcompounds. Also included are group IV semiconductors such as silicon orgermanium, and organic semiconductors. Nanocrystals are capable ofemitting electromagnetic radiation upon excitation. The color of theemitted light depends on the size of the crystal and the material. Thelarger the crystal, the more red the output, and the wider the emissionband. Nanocrystals generally have narrow emission bands, i.e. thewavelength band of emission does not exceed about 40 nm in the visibleand preferably does not exceed about 20 nm. The width of the emissionband scales with energy, not wavelength. Nanocrystals also generallyhave a broad absorption band, i.e. the electromagnetic radiationabsorption continuously increases from the onset which occurs near tobut at slightly higher energy than the emission band. The growth ofcore/shell semiconductor nanocrystals is described in Xiaogangi Peng etal., “Epitaxial Growth of Highly Luminescent CdSe/CdS Core/ShellNanocrystals with Photostability and Electronic Accessibility, ” J. Am.Chem. Soc. 1997, 119, 7019-7029. U.S. Pat. No. 5,505,928 to Alivisatoset al. describes the preparation of III-V semiconductor nanocrystals.U.S. Pat. No. 5,262,357 to Alivisatos et al. describes formation of thinfilms from nanocrystal precursors. U.S. Pat. No. 5,751,018 to Alivisatoset al. describes the bonding of semiconductor nanocrystals to solidsurfaces.

U.S. Pat. No. 5,537,000 to Alivisatos et al. describes anelectroluminescent device having an electron transport layer ofsemiconductor nanocrystals. The device has a hole injection layer, ahole transport layer, the nanocrystal electron transport layer, and anelectron injection layer. Device output color is controlled by voltageas well as nanocrystal size and type. A flat panel display is producedfrom an array of the electroluminescent devices.

The use of luminescent semiconductor nanocrystals in biological probesis described in U.S. patent application Ser. No. 08/978,450.

SUMMARY OF THE INVENTION

The invention is a multi-color electronic display which utilizes arraysof luminescent semiconductor nanocrystals for pixel elements. Each pixelor addressable color element is formed of a number of suitably sizednanocrystals to produce a desired color. While the display can be basedon the three primary colors, red, green and blue, greater flexibility inusing many more different colors by selecting suitably sizednanocrystals to produce different colors is possible. In addition to thepixel array formed of the nanocrystals, the display includes a pixeladdressing (including optical pumping) system. In one illustrativeembodiment, a backlight source with a pixelated array of elements isused. In a second illustrative embodiment, a liquid crystal modulator isused to modulate a backlight. In a third illustrative embodiment, amodulated laser is raster scanned over the pixel array. Light of asingle wavelength can be used to excite the nanocrystals of all colors.Ultraviolet or blue light sources are preferred. If UV backlight isused, all the pixels contain the appropriate sized nanocrystals;however, if blue excitation is used, then the blue pixels can contain nonanocrystals and merely pass the backlight directly. A long-pass filterhaving the appropriate wavelength typically covers the pixel array.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A-G are simplified cross-sectional views of different embodimentsof an optically pumped luminescent semiconductor nanocrystal basedelectronic display with three types of pixel addressing systems.

FIG. 2A illustrates one excitation scheme based on UV excitation.

FIG. 2B illustrates an alternate excitation scheme based on blueexcitation.

DETAILED DESCRIPTION OF INVENTION

The invention comprises a color display which is based on theluminescence phenomena in semiconductor nanocrystals. It takes advantageof the following properties: (1) color tunability of the luminescence isobtained by quantum confinement through size control; (2) theluminescence of such nanocrystals is very narrow, i.e. spectrally pure;(3) the luminescence quantum efficiency of core-shell nanocrystals isvery high; (4) the nanocrystals are photostable; (5) the absorptionlineshape of the nanocrystals continues above an onset (which depends onparticle size). Therefore, the luminescent semiconductor nanocrystalsare capable of being excited over a broad bandwidth, yet exhibitemission in a narrow wavelength band. Thus, UV or blue light (UV/blue)as well as other color light may be used to excite different sizenanocrystals with different emission spectra. For example, the sameUV/blue light can be used to excite blue, red and green pixels to getthe three needed primary RGB display colors with a single excitationsource. Alternatively nanocrystals with any other colors can be used.More than just three colors can be used, i.e. any number of displaycolors, e.g. 25, could be used. While the invention is describedprimarily with reference to RGB pixels illuminated by UV or blue light,the principles of the invention apply to any color nanocrystalsilluminated by any suitable color source.

As used herein, the term pixel refers to an independently addressablesingle color element, i.e. a group of red emitting nanocrystals whichform a red color element on the display are referred to as a red pixel.The term pixel could be used to describe a combined element including acolor element of each of the colors, e.g. a pixel with red, green andblue dots or subpixel elements. However, since each of the colorelements must be separately addressable, it is simpler to refer: to eachcolor element as a pixel.

The display can be based on many different pixel addressingtechnologies. As used herein, the pixel addressing system includes boththe excitation or optical pumping source and the means for addressing orexciting individual pixels in the array in a selected pattern. Themodulation of the light for the display purpose can be achieved bydirect modulation of a backlight source if it is pixelated and eachpixel can be addressed and turned “on” and “off” by electronic means(FIGS. 1A-C, G). That will be the case, for example, for an array ofindividual UV/blue light emitting diodes (LEDs) or semiconductor lasersin the UV/blue region (based, for example, on GaN/InGaN technology) or afabricated array of addressable two-dimensional LEDs or vertical cavitysurface emitting lasers (VECSELs). The back light can also be modulatedexternal to the source, as in the case of a liquid crystal display,where the backlight will be modulated when passing through the liquidcrystal (FIGS. 1D-E). Pixels can also be addressed by a single modulatedlaser beam which is raster scanned across the display, similar torastering an electron beam in a CRT (FIG. 1F).

Instead of using color filters as in conventional LCDs, the screen“dots” are made of luminescent semiconductor nanocrystals. Each R, G, orB pixel, for example, contains single-sized, very high quantum-yield,very stable, core-shell nanocrystals with a very narrow emissionspectrum (approx. 20-40 nm) in the red, green or blue. The nanocrystalscan be deposited on a glass or other transparent panel by varioustechniques, e.g. by direct printing by ink-jet technology, or bychemical bonding through specific glass and nanocrystal surfacederivitization and patterning techniques, or by embedding nanocrystalsin a polymer film and then patterning. Nanocrystals can also bedissolved in a liquid. Pixelation in this case is achieved by etchingmicro-wells into the panel and sealing individual size nanocrystals insolution into different wells. The concentration/optical density ofnanocrystals in each dot is optimized such that the excitation light isefficiently absorbed to provide bright emission from the nanocrystalswith minimal reabsorption.

Illustrative embodiments of an optically pumped luminescentsemiconductor nanocrystal based display (showing three pixels), withthree different pixel addressing systems, are shown in FIGS. 1A-G. Theelectronic displays 10 a-f of FIGS. 1A-F include a plurality or array 12of nanocrystals 14, 18, 22 which form a plurality of pixels 16, 20, 24of different colors. As shown the plurality or array 12 of nanocrystalsinclude a group of red light emitting nanocrystals 14 which tin form ared pixel 16, a group of green light emitting nanocrystals 18 which forma green pixel 20, and a group of blue light emitting nanocrystals 22,which form a blue pixel 24. Display 10 g of FIG. 1G has a blue pixel 24without nanocrystals. While an electronic display based on the threeprimary colors RGB is illustrated, the nanocrystal approach providesgreat flexibility in using virtually any number of different of colorssince the colors can easily be obtained by selecting the proper sizednanocrystals to produce a desired color. The nanocrystal array 12 may besandwiched between a glass or other transparent plate 26 and long-passfilter 28 (FIGS. 1A, D, E, F). The nanocrystals can be deposited, assolids or even in a liquid medium, on glass panel 26 in a number ofdifferent ways, including but not limited to the exemplary techniquesdescribed above. Alternately, the nanocrystal array 12 and plate 26 canbe reversed (FIGS. 1B, G), or plate 26 can be omitted (FIG. 1C), orfilter 28 can be omitted (FIG. 1G).

While the nanocrystal array 12 is similar in the display embodiments10a-g of FIGS. 1A-G, the pixel addressing systems (including opticalpumping sources) 40 a-c are of three different types. In FIG. 1A,backlight source 30 a is formed of a plurality or array of addressableor individually and independently operable source elements 42 a, b, cwhich correspond to an associated pixel 16, 20, 24 respectively. Thesource elements 42 a, b, c are typically UV or blue LEDs orsemiconductor lasers (the schematic representations include associatedelectronics). As source elements 42 a, b, c are turned on and off, theassociated pixels 16, 20, 24 are turned on and off, producing a colordisplay. To avoid diffraction effects, backlight source 30 a should beas close to nanocrystal array as possible, i.e. contacting panel 26(distance D=0).

Variations in displays using addressing system 40 a are shown in FIGS.1B, C. In FIG. 1A backlight source 30 a is separated from nanocrystalarray 12 by plate 26. In order to get array 12 even closer to source 30a to reduce diffraction effects, array 12 and plate 26 can be reversed,as shown in FIG. 1B. Display 10 b has array 12 deposited on plate 26,but backlight source 30 a is up against the array 12 side of plate 26instead of the opposite side. Since array 12 is in contact withbacklight source 30 a, it may be possible to deposit array 12 directlyon source 30 a, and omit plate on 26 altogether, to from display 10 c,as shown in FIG. 1C.

In FIG. 1D, display 10 d has an addressing system 40 b in whichbacklight source 30 b is a single source. Light from backlight source 30b is modulated by a liquid crystal modulator 44 positioned betweensource 30 b and nanocrystal array 12. Light from backlight source 30 b,which is typically an ultraviolet or blue light source, passes throughpolarizer 34 into the liquid crystal 32. Liquid crystal 32 is modulatedby electrical signals applied on contacts 36 on opposed faces thereof,to produce the appropriate patterns of excitation signals to excite thenanocrystal array 12. The modulated excitation light from the liquidcrystal 32 passes through an analyzer 38 which is a polarizer with itsaxis orthogonal to that of polarizer 34 and through glass plate 26 intothe nanocrystal array 12. When the part of liquid crystal 32 whichcorresponds to a pixel is off, light passing through polarizer 34 isblocked by analyzer 38. When the liquid crystal 32 is turned on, theliquid crystal rotates the polarization of the light passing through sothat the light is passed by analyzer 38 to array 12 to excite the pixel.If soure 30 b is polarized, then polarizer 34 can be omitted. Theposition of array 12 and plate 26 can also be reversed as in FIG. 1B.FIG. 1E shows a display 10 e which is similar to 10 d. Backlight 30 b isa slab or waveguide 46 which is end pumped by pump source 48 and emitslight along a lateral side.

In FIG. 1F, the optical pumping source (addressing system) 40 c ofdisplay 10 f is a single laser 30 c which can be modulated and rasterscanned across the nanocrystal array 12. Thus as the output beam oflaser 30 c sweeps across the nanocrystal array 12 in a pattern, theoutput beam is turned on and off depending on whether a particular pixel16, 20, 24 is to be on or off during that particular scanning cycle.

FIG. 1G shows a display 10 g which is similar to display 10 b, but whichutilizes the excitation scheme of FIG. 2B. Blue pixel 24 is formed as anopen space without nanocrystals and passes blue light from source 30 a.No filter 28 is required over the blue pixels

Although illustrated with these three optical pumping systems, variousother optical pumping schemes can also be used. Any color excitationlight source can be used. The only requirement is that the light sourcehave higher photon energy compared to the color of the display. Aninfra-red (IR) displaycould be produced.

Various backlight excitation schemes can also be used, e.g. asillustrated in FIGS. 2A, B. If. UV backlight excitation is used (schemeA, FIG. 2A), all 3 RGB pixels contain the appropriate size nanocrystals.If blue excitation is used (scheme B, FIG. 2B) the blue pixel willcontain no nanocrystals and pass the backlight directly (FIG. 1G). Inscheme A, a long-pass filter with the appropriate cut-off wavelength(pass RGB but not UV excitation light) will pass the emitted light fromthe nanocrystals and block unwanted excitation light from passingthrough. In this case the filter will cover all pixels. In scheme B anappropriate long pass filter (pass RG but not blue light) will beimplemented only for green and red pixels (no filter for blue pixels).If the optimal optical density is large enough (OD about 2), no filtersare needed because the backlight will be entirely absorbed by thenanocrystals.

This invention makes very efficient use of the backlight since thephoto-luminescence quantum efficiency is very high and there are noabsorbing filters. Optical pumping of nanocrystals is much superior toelectrical pumping (electroluminescence) and should provide for veryefficient, bright and vivid colors. The spectral purity of the emissionand ability to control the color of emission allows the display to spanthe whole chromaticity diagram and therefore obtain any desired colorwith high quality.

In the case of a liquid crystal display implementation, the use ofpolarized UV/blue laser backlight can eliminate one of the sheetpolarizers, making the device even more efficient.

Other advantages of optically-pumped nanocrystal based displays are in:(1) the ease and simplicity of fabrication; (2) they can be used forflexible and large area displays; (3) they are compatible with very highresolution displays (especially when pumped with UV/blue light); (4)high extinction coefficients can be achieved with thin films; (5) verygood size distribution will eliminate self absorption because of Stokesshift.

Changes and modifications in the specifically described embodiments canbe carried out without departure from the scope of the invention whichis intended to be limited only by the scope of the appended claims.

1. A multicolor display apparatus comprising: a nanocrystal arrayforming an array of pixels, wherein the array of pixels comprises afirst discrete pixel comprising first nanocrystal particles of a firstsize and a second discrete pixel comprising second nanocrystal particlesof a second size; and a pixel addressing system operatively associatedwith the array of pixels, wherein the pixel addressing system is adaptedto excite the first nanocrystal particles in the first discrete pixelusing light comprising a wavelength to cause the first pixel to producea first color and to excite the second nanocrystal particles in thesecond discrete pixel using light of the same wavelength to cause thesecond discrete pixel to produce a second color.
 2. The apparatus ofclaim 1 wherein the pixel addressing system includes a light source thatproduces only one wavelength of light.
 3. The apparatus of claim 1wherein the pixel addressing system includes a backlight source and aliquid crystal modulator.
 4. The apparatus of claim 1 wherein the lightis blue light.
 5. The apparatus of claim 1 further comprising a longpass filter in operative association with the array of pixels.